Debt Short-term borrowings (Details 2) (Short-term debt [Member], Revolving credit agreement [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Cascade Natural Gas Corporation [Member]
Short-term debt [Line Items]
Short-term debt, weighted average interest rate	3.30%
Ratio of total debt to total capitalization as specified in debt convenants.	65.00%
Intermountain Gas Company [Member]
Short-term debt [Line Items]
Short-term debt, weighted average interest rate	2.30%
Ratio of total debt to total capitalization as specified in debt convenants.	65.00%
Conditions resulting in an early termination date under any swap contract that is in excess of this amount	10.0

Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of short-term debt outstanding by type or by instrument at that time.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Subparagraph b

-Article 5

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant